Emissivity corrected optical pyrometer

ABSTRACT

An optical pyrometer for measuring true surface temperature utilizes a spherical light integrator having a reflective internal surface. Light from the subject surface, in traveling an optical path into the spherical integrator, is modulated at a frequency fc. The pyrometer employs an internal light source that is modulated at a frequency fL. The modulated light from the internal source can be directed onto the subject surface to permit a measurement to be made to ascertain the reflectance of the subject surface. The reflected light from the internal source, because of its modulation fL, is distinguishable from the light otherwise emanating from the subject surface which is modulated only at frequency fc in traversing the optical path to the integrator. An optical filter is employed to filter the light emerging from a port in the spherical integrator and a photosensitive detector is disposed to have the filtered light incident upon it.

United States Patent Aisenberg [451 Oct. 17, 1972 [54] EMISSIVITYCORRECTED OPTICAL PYROMETER [72] lnventor: Sol Aisenherg, Natick, Mass.

[73] Assignee: Whittaker Corporation, Waltham,

Mass.

[22] Filed: Jan. 18, 1971 [21] Appl. No.: 106,980

[52] US. Cl. ..3S6/48, 73/355, 250/227, 250/228 [5 l] Int. Cl ..G0lj5/52, GOlj 5/62 [58] Field of Search ..356/43, 50; 250/228, 227;

[56] References Cited UNITED STATES PATENTS 3,057,200 10/1962 Wood..356/43 2,439,373 4/1948 Stearns, Jr. ..250/228 Primary Examiner-RonaldL. Wibert Assistant Examiner-V. P. McGraw Attorney-W0lf, Greenfield &Sacks and Donald E. Nist [5 7] ABSTRACT An optical pyrometer formeasuring true surface temperature utilizes a spherical light integratorhaving a reflective internal surface. Light from the subject surface, intraveling an optical path into the spherical integrator, is modulated ata frequency f The pyrometer employs an internal light source that ismodulated at a frequency f,,. The modulated light from the internalsource can be directed onto the subject surface to permit a measurementto be made to ascertain the reflectance of the subject surface. Thereflected light from the internal source, because of its modulation f,,,is distinguishable from the light otherwise emanating from the subjectsurface which is modulated only at frequency f in traversing the opticalpath to the integrator. An optical filter is employed to filter thelight emerging from a port in the spherical integrator and aphoto-sensitive detector is disposed to have the filtered light incidentupon it.

5 Claims, 3 Drawing Figures LIGHT MODULATOR INVENTOR SOL AISENBERG BYmay 21W; 5%

ATTORNEYS FIELD OF TI-H'E INVENTION The invention relates in general toapparatus for the accurate measurement of high temperature byobservation of the light emitted from heated substances. Moreparticularly, the invention pertains to an optical pyrometric device inwhich the true surface temperature of a hot material is ascertained bycorrecting for the materials surface emissivity.

BACKGROUND OF THE INVENTION In the measurement of the true surfacetemperature of an incandescent material by observation of the lightemitted from the material, it is known that the emissivity 6 of thesurface of the material affects its apparent brightness. To obtain thetrue surface temperature by optical pyrometry, it has been customary toascertain the reflectance R, of the subject surface and to determine thesurface emissivity through the relationship Fl R,. The apparent surfacetemperature is then corrected for surface emissivity. The procedures nowused to ascertain true surface temperature by optical pyrometrytechniques are tedious and time consuming.

OBJECTS OF THE INVENTION The principal object of the invention is toprovide optical pyrometn'c apparatus which facilitates the rapid 3determination of true surface reflectance. A further objective of theinvention is to provide an optical pyrometric system capable of directlyproviding a measurement of true surface temperature.

THE INVENTION The invention resides in an optical pyrometer having aninternal light source and means for directing light from that sourceupon the subject surface. The apparent surface temperature is measuredwithout employing the internal light source. A measurement is then madein which light from the internal source is reflected from the subjectsurface. The light from the internal source is modulated in a mannerpermitting it to be distinguished from the radiations otherwise emittedfrom the subject surface.

THE DRAWINGS THE EXPOSITION Referring now to FIG. 1 of the drawings, thepreferred arrangement of the invention is shown in diagrammatic form.Situated within housing 1 is an integrating sphere 2 having a reflectiveinternal surface. Light emitted from the surface 3 of a hot material isdirected upon the reflective internal surface of the integrating sphereby a plurality of light transmitting pipes.4 which are here collectivelytermed the fiber optics." The light entering the sphere from the fiberoptics is integrated by the multiple reflections which occur within thesphere. The light from the hot material which passes through .theaperture in housing 1 is periodically permitted to impinge upon thefiber optics by an incident light chopper 5. The incident light chopper,in a simple form, is constituted by a motor which causes an opaque plate5A to rotate in front of the fiber optics. The plate has a plurality ofslits in it which, upon rotationof the plate, periodically pass beforethe fiber optics to permit light to be transmitted to the integratingsphere at a frequency f,.

The integrating sphere 2 has an aperture 6 in its wall which permits theintegrated light to proceed to an optical'filter 7. The light passingthrough filter 7 impinges upon a photosensitive detector 8 which emitsan electrical signal in response to the light incident upon thedetector. The optical filter, as is customary in the field ofpyrometrics, permits only light which is within a relatively narrow bandof the spectrum to impinge upon the detector.

Protruding centrally through integrating sphere 2 and the fiber optics 4is a tube for directing light from an internal source 10 toward theheated material whose surface temperature is being ascertained. Thelight 0 from source 10 is also directed into the integrating sphere,after being chopped by a reference chopper 1 1 which chops the light ata frequency f,; that is different from the frequency f}, of the incidentlight chopper 5. The reference chopper 11 is, in FIG. 1, indicated by anopaque disc 11A having in it a plurality of slits. The light from source10 is controlled in intensity at a frequency f,, by a modulator 12.

The signal from detector 8 is, as indicated in FIG. 2, coupled to theinput of a low noise preamplifier 13. The output of preamplifier 13 iscoupled to the inputs of tuned amplifiers 14, 1S and 16. Amplifier 14 istuned to respond to signals of frequency f amplifier -l5 is tuned torespond to signals of frequency f,,+f,, and amplifier 16 is tuned torespond to signals of frequency f,,+f The tuned amplifiers 14, 15, 16are preferably operational amplifiers and the output signals of thepreamplifier is fed into the input of each of those amplifiers. Theoutput of amplifier 14 is demodulated by a demodulator 17 to obtain aDC. signal V(f,,-) whose amplitude is a measure of the surface radiationof the heated material. Similarly, the output of tuned amplifier 15 isapplied to a demodulator 18 whose output is a signal V(f,,+f which is ameasure of surface reflectance of the heated material. The output oftuned amplifier 16 is similarly demodulated by a demodulator 19 whoseoutput provides a signal F(f +f The outputs of the demodulators arecombined in a computational device, here represented as a computer 20,whose output 21 is a measure of the true surface temperature of theheated subject.

In the operation of the invention, the surface radiation of the heatedsubject is measured using the inface temperature of the subject,uncorrected for surface emittance. The surface emittance e isascertained by measuring the surface reflectance R, of the subject,since it is known that e lR,. To obtain the surface reflectance R,,another measurement is made in which the internal light source isemployed. The intensity of internal light source is modulated atfrequency L, by modulator 12. The light from source 10 is directedthrough tube 9 and impinges at the chopping frequency f upon the surfacewhose reflectance is to be ascertained. The modulated light which isreflected from the surface of the subject is, hence, chopped atfrequency f,,. The reflected light which impinges upon detector 8,therefore, has an intermodulation frequency f whereas the lightotherwise emanating from the surface has the frequency f To provide forinternal calibration, light from internal source 10 is chopped by thereference chopper 11 at frequency f while the intensity of that light ismodulated at frequency f by modulator 12. The light at theintermodulation frequency f +f is introduced into integrating sphere 2and is reflected from the surface of the sphere through the aperture 6.The light is then filtered by filter 7 and impinges upon detector 8.

The reflectance R, of the subject hot surface is related to thereflectance R of the integrating sphere by (fL+ f0) R,= R

K1 vowing where V(f,,+fl) and V(f,,+fn) are voltages proportional to thetwo reflected light intensities and K is a calibration constant. Thisequation can be simplified to maid W HBL.

where K =K R is a constant set during the calibration of the pyrometeragainst a standard.

From the signal voltage V(f which is proportional to surfacereflectance, and the internal calibration reflectance signal voltageV(f,,+fn), the true surface temperature T, can be calculated from thefunction Since and Ts=Ka 1/4 where K, is a constant.

The computer for calculating the function T can, in a simple form, berepresented by the arrangement of FIG. 3 which employs a divisioncircuit 22 having as its inputs the signals I (n,+f,). VU -i'fn) andhaving the calibration constant K, preset by a potentiometer. The outputof circuit 22 is fed into a subtraction circuit 23 to obtain a signalrepresenting the surface emittance e.

are related to the radiation from the source K V(f and the determinationof the surface emittance e. The fractional error 86/6 in the emittanceis related to the fractional error in the reflectance SRJR, by

Assuming a reflectance of about 9() percent, a l percent error in Rresults in a 9 percent error in 5, since The error in reflectance isproduced by an error in the ratio (fL +13 l iifcislsm,

and by an error in the internal reflectance R If the error in R is about1 percent, then the r.m.s. error in R, is 1.4 percent. The correspondingerror in e is 12.6 percent. Upon taking the fourth root, thecontribution to the error in T, is about l.9 percent. To this error mustbe conjoined the expected error in [V(f,)] which, by careful control canbe held to about 1 percent. Thus, an r.m.s. error in T, of between 2percent and 3 percent can be expected. I

In view of the numerous forms which embodiments of the invention cantake, it is not intended that the scope of the invention be restrictedto the precise structure and arrangement illustrated in the drawings ordescribed in the exposition. For example, the optical system,illustrated in the FIG. 1 embodiment, which directs the light from theemitting surface to the integrating sphere, can be replaced by a lenssystem or collimating tube to perform the same function. Further, themechanical light chopping devices can be replaced by other types ofshutters or light modulating devices. It is intended, therefore, thatthe scope of the invention be delimited by the claims appended heretoand that within that scope be included only those structures which inessence utilize the invention.

What is claimed is:

1. A pyrometer for measuring the true surface temperature of a subjectsurface, the pyrometer comprising a light integrator having a reflectiveinternal surface;

means providing an optical path from the subject surface to thereflective internal surface of the inte grator;

means for modulating at a frequency f light propagating along theoptical path;

an internal light source;

means for modulating the internal light source at a frequency f,,;

means for directing light from the internal source modulated atfrequency f,, upon the subject surface whereby reflections of that lighttravel along 7 a first amplifier tuned to respond to signals offrequencyf a second amplifier tuned to respond to signals of frequency f,+f,,,

a third amplifier tuned to respond to signals of frequency f,,+fm andmeans for coupling the signal output of the photosensitive detector tothe input of the first, second and third amplifiers.

4. The pyrometer according to claim 3, further comprising demodulatorscoupled to the outputs of the first,

second and third amplifiers for demodulating the outputs of theamplifiers, the demodulators' providing the outputs VOL), V(fL+f andV(fi,+f 5. The pyrometer according to claim 4, further comprisingcomputational apparatus responsive to the outputs of the demodulators,the computation apparatus providing an output representing the truesurface temperature T, of the subject surface according to the function(fL+fR) where K and K are constants.

1. A pyrometer for measuring the true surface temperature of a subjectsurface, the pyrometer comprising a light integrator having a reflectiveinternal surface; means providing an optical path from the subjectsurface to the reflective internal surface of the integrator; means formodulating at a frequency fc light propagating along the optical path;an internal light source; means for modulating the internal light sourceat a frequency fL; means for directing light from the internal sourcemodulated at frequency fL upon the subject surface whereby reflectionsof that light travel along said optical path to the internal surface ofthe integrator; an optical filter disposed to filter the light emergingfrom a port in the integrator; and a photo-sensitive detector disposedto have the filtered light incident upon it.
 2. The pyrometer accordingto claim 1, further comprising means for modulating at a frequency fRlight from the internal source; and means for causing light from theinternal source which has been modulated at frequencies fL + fR toimpinge upon the reflective internal surface of the integrator.
 3. Thepyrometer according to claim 2, further comprising a first amplifiertuned to respond to signals of frequency fc, a second amplifier tuned torespond to signals of frequency fL+fc, a third amplifier tuned torespond to signals of frequency fL+fR, and means for coupling the signaloutput of the photo-sensitive detector to the input of the first, secondand third amplifiers.
 4. The pyrometer according to claim 3, furthercomprising demodulators coupled to the outputs of the first, second andthird amplifiers for demodulating the outputs of the amplifiers, thedemodulators providing the outputs V(fc), V(fL+fc) and V(fL+fR).
 5. Thepyrometer according to claim 4, further comprising computationalapparatus responsive to the outputs of the demodulators, the computationapparatus providing an output representing the true surface temperatureTs of the subject surface according to the function